Electroneutral Na⁺/Cl^- cotransport activity of zebrafish Slc12a10.1 expressed in Xenopus oocytes

Na⁺/Cl^- cotransporter 2 (Ncc2 or Slc12a10) is a membrane transport protein that belongs to the electroneutral cation-chloride cotransporter family. The Slc12a10 gene (slc12a10) is widely present in bony vertebrates but is deleted or pseudogenized in birds, some bony fishes, and most mammals. Slc12a10 is highly homologous to Ncc (Slc12a3 or Ncc1); however, there are only a few reports measuring the activity of Slc12a10. In this study, we focused on zebrafish Slc12a10.1 (zSlc12a10.1) and analyzed its activity using Xenopus oocyte electrophysiology. Analysis using Na⁺-selective microelectrodes showed that intracellular sodium activity (aNa_i) in zSlc12a10.1 oocytes was significantly decreased in Na⁺- or Cl^--free medium and recovered when Na⁺ or Cl^- was readded to the medium. Similar analysis using a Cl^--selective microelectrode showed that intracellular chloride activity (aCl_i) in zSlc12a10.1 oocytes significantly decreased in Na⁺- or Cl^--free medium and recovered when Na⁺ or Cl^- was readded to the medium. When a similar experiment was performed with a voltage clamp, the membrane current did not change when aNa_i of zSlc12a10.1 oocytes was decreased in Na⁺-free medium. Molecular phylogenetic and synteny analyses suggest that gene duplication between slc12a10.2 and slc12a10.3 in zebrafish is a relatively recent event, whereas gene duplication between slc12a10.1 and the ancestral gene of slc12a10.2/slc12a10.3 occurred at least about 2 million years ago. slc12a10 deficiency was observed in species belonging to Ictaluridae, Salmoniformes, Osmeriformes, Batrachoididae, Syngnathiformes, Gobiesociformes, Labriformes, and Tetraodontiformes. These results indicate that zebrafish Slc12a10.1 is an electroneutral Na⁺/Cl^-cotransporter and establish its evolutionary position among various teleost slc12a10 paralogs.NEW & NOTEWORTHY Na⁺/Cl^- cotransporter 2 (Slc12a10; Ncc2) is a protein highly homologous to Ncc (Slc12a3; Ncc1); however, there are only a few reports measuring the activity of Slc12a10. Electrophysiological analysis of Xenopus oocytes expressing zebrafish Slc12a10.1 showed that Slc12a10.1 acts as an electroneutral Na⁺/Cl^-cotransporter. This is the third report on the activity of Slc12a10, following previous reports on Slc12a10 in eels.